1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) which employs a voltage control birefringence method for controlling tilt directions of liquid crystal by using an electric field, and in particular to a driving circuit of such an LCD.
2. Description of the Prior Art
LCDs, in which liquid crystal is enclosed between a pair of substrates and a voltage is applied to the enclosed liquid crystal for desired display, are advantageously small and thin, and the power consumption thereof can be easily reduced. Due to these advantages, LCDs are widely used as displays in various office automation equipment, audio visual equipment, and portable or on-board devices.
In particular, a DAP (deformation of vertically aligned phase) LCD is proposed, which includes liquid crystal with negative dielectric constant anisotropy, and controls initial alignment of the liquid crystal molecules to be vertically-aligned by using a vertical alignment layer. Specifically, a DAP LCD employs one type of electrically controlled birefringence (ECB) methods, and controls transmittance and displayed colors of the light coming into the liquid crystal layer by utilizing a difference in a reflective index between the longer and shorter axes of a liquid crystal molecules, i.e., a birefringence phenomenon. A pair of substrates are provided each with a polarization plate attached on the outer surface thereof, such that their polarization directions are orthogonal to each other. When voltage is applied to the liquid crystal layer, linearly polarized light which has been introduced into the liquid crystal layer via the polarization plate on one side of the substrate is converted into elliptically or circularly polarized light due to birefringence of the liquid crystal layer, and is partly ejected from the polarization plate on the other side. Since the extent of birefringence of the liquid crystal layer, i.e., a phase difference (a retardation amount) between ordinary and extraordinary ray components of the incoming linearly polarized light, is determined according to the voltage applied to the liquid crystal layer, i.e., the intensity of an electric field caused in the liquid crystal, the amount of ejected light from the second polarization plate can be controlled for every pixel by controlling for every pixel the voltage applied to the liquid crystal layer. This eventually makes it possible to display a desired image as a whole.
A DAP LCD is originally superior in light transmittance as it utilizes birefringence, and can be made without a rubbing step in a manufacturing process as a result of improvement of a panel structure, or the like, of the display. Moreover, the display can have an improved viewing angle. However, when voltage is applied to vertically-aligned liquid crystal, tilt directions of the respective liquid crystal molecules may vary even though the tilt angles thereof are the same. Therefore, it takes some time until the liquid crystal molecules within one pixel region come to have the same tilt directions. This may problematically slow a response time of liquid crystal molecules with respect to the applied voltage.
The present invention has been conceived to overcome the above problems and aims to provide a driving circuit of an LCD which controls vertical and horizontal components for liquid crystal alignment by using an electric field. The driving circuit can improve response time of liquid crystal.
In order to achieve the above object, according to one aspect of the present invention, there is provided a liquid crystal display for controlling, by using an electric field, vertical components and horizontal components for alignment of liquid crystal sandwiched by a first substrate and a second substrate having electrodes for driving the liquid crystal, a lower limit value of a range for a driving voltage for application to the liquid crystal being set greater than 0 V. Preferably, the lower limit of a driving voltage to be applied to the liquid crystal is set at an optical characteristics changing voltage or over.
Further, in the above liquid crystal display, the lower limit value of the range for a driving voltage for application to the liquid crystal is set to so as to change while following a change, due to a temperature change, in an optical characteristics change voltage for temperature-dependent liquid crystal.
Still further, in the above liquid crystal display, the lower limit value of the range for a driving voltage for application to the liquid crystal is set so as to achieve a display contrast of 50 or over.
Still further, in the above liquid crystal display, the liquid crystal is set to have initial vertical or horizontal alignment having an initial tilt angle of approximately 0xc2x0.
Still further, in the above liquid crystal display, the electrode has an electrode-free region in a predetermined shape for controlling plan-direction components for the alignment of the liquid crystal by using a diagonal electric field caused at an edge of the electrode.
Still further, in the liquid crystal display, the liquid crystal is vertically-aligned.
According to another aspect of the present invention, in the liquid crystal display, the electrode for driving the liquid crystal formed on the first substrate includes a number of pixel electrodes provided in a matrix thereon; the number of pixel electrodes are connected to corresponding polyxe2x80x94Si thin film transistors each using a polyxe2x80x94Si layer formed at a low temperature for an active layer; and the liquid crystal is driven for each pixel electrode between the number of pixel electrodes and common electrodes for driving the liquid crystal formed on the second substrate.
For application to a color LCD, the upper limit of a voltage for driving the liquid crystal may be determined respectively for R, G, B for individual control of the transmittance of R, G, B components.
As described above, according to the present invention, it is possible to increase a response time of liquid crystal while maintaining high contrast by setting a range of a driving voltage to be applied to a liquid crystal layer at a value larger than 0 V. Also, high-quality display can always be achieved at various temperatures with an arrangement in which the lower limit of a voltage to be applied to the liquid crystal layer for an OFF display, which is set at a value equal to or above the optical characteristics changing voltage, is arranged to change while following the change of the optical characteristics changing voltage according to the ambient temperature of the display.